1. Field of the Invention
The present invention relates to a mixture thin film forming apparatus, and more particularly, it relates to an apparatus for forming a thin film through a solid raw material, such as an oxide high-temperature superconducting thin film, for example, which is gasified at a relatively high temperature.
2. Description of the Prior Art
As disclosed in Japanese Patent Publication Gazette No. 2668/1989, for example, it has been pointed out that the raw material gas is precipitated on a cooled portion by thermal conduction loss or the like when a solid raw material which is gasified at a high temperature of 150.degree. to 200.degree. C. is employed for this type of apparatus. This literature takes up a gas introducing part for a reaction pipe, and proposes a method of preventing cooling of the gas introducing part caused by thermal conduction loss. In such case of employing a single raw material, a gas passage for the gas introducing part has been simple and it has been unnecessary to pay much attention to the temperature except for the gas introducing part.
In a compound thin film forming apparatus represented by that for forming a GeAs thin film, on the other hand, a gas passage of its gas supply system is extremely complicated for the following reason, as disclosed in Japanese Patent Laying-Open Gazette No. 90121/1988, for example:
When a solid raw material is sublimated for employment as a raw material gas, the amount of supply of the gas is controlled by the flow rate of a carrier gas which is controlled by a mass flow controller or the like. In order to control the amount of the raw material gas, therefore, it is necessary to maintain the raw material gas, which is contained within a constant volume of the carrier gas, in a constant amount. Therefore, it is necessary to maintain the gas pressure in a sublimation chamber constant in order to eliminate time change of the amount of the gas which is sublimated and generated from the solid raw material. Consequently, the gas passage is complicated.
Such a conventional compound thin film forming apparatus has been developed while taking into consideration the case of employing a sublimable solid raw material such as trimethylgallium, for example, which can sufficiently take vapor pressure at the room temperature, and not much attention has been paid to temperature control of portions other than the gas supply system, particularly the sublimation chamber.
On the other hand, a recently discovered solid material for forming a thin film of an oxide high-temperature superconductor is hardly gasified around the room temperature, and a high temperature of at least 100.degree. C. is required for gasifying this material. In order to employ such a solid raw material, it is necessary to prevent precipitation of the raw material gas by maintaining the gas supply system at a temperature higher than the gasification temperature. Further, it is also necessary to prevent the gas supply system from abnormal temperature rise, to thereby prevent irregular reaction of the raw material gas. Thus, the gas supply system must be controlled within a prescribed temperature range.
However, respective components of the gas supply system are different in heat capacity from each other. If the temperatures of the components of the gas supply system are commonly controlled by a single temperature control part, therefore, large temperature differences are caused between the components. As the result, it is impossible to control the overall gas supply system within the aforementioned prescribed temperature range, and hence precipitation is caused by low temperatures or irregular reaction etc. is caused by high temperatures. Thus, it has been impossible to stably supply the gas.